Analysis of communication pathways during seed development in Arabidopsis thaliana

Abstract

Seed development in flowering plants requires coordination between the two genetically different fertilization products, the embryo and the endosperm and the surrounding maternal tissues, the integuments. However, little is known about the coordination of endosperm and embryo growth. In Arabidopsis, mutations in the cell cycle regulator CYCLIN DEPENDENT KINASE A;1 (CKDA;1) result in pollen that only successfully fertilizes the egg cell and seeds generated from crosses with cdka;1 pollen develop endosperm with solely maternal contribution. Here, fertilization by the cdka;1 mutant pollen was used to dissect early seed development. Crosses of 14 Arabidopsis accessions pollinated with cdka;1 mutant pollen revealed a large natural genetic variation with regard to the development of endosperm without paternal contribution. This work revealed a surprisingly large degree of autonomy in embryo growth, but also showed the embryo’s growth restrictions with regard to endosperm size. By using a recombinant inbred line population between the two Arabidopsis accessions Bayreuth-0 and Shahdara four QTLs were discovered, two main and four complex loci that influence the development of unfertilized endosperm. The genes of two DNA N-glycosylases ROS1 and DME, which catalyze the demethylation of symmetrical cytosine methylation, lay inside the two intervals of one of the two complex QTLs. A functional analysis revealed a new aspect of ROS1 and DME in restricting the proliferation of unfertilized endosperm. Moreover, ros1-dme double mutants could rescue the observed seed abortion upon cdka;1 pollination dependent on the activity of the methyltransferase MET1 during the sporophytic phase. Surprisingly, the rescue was independent of dme co-transmission, indicating an effect of ros1 together with dme in trans. The inheritance pattern of the mutant phenotypes revealed a paramutation-like phenomenon and the detection of almost 100% relative methylation levels on PHE1 and AGL36 sequence loci in ros1-dme double compared to the single mutants and Col-0 wild-type plants suggested that hypermethylation caused the mutant phenotype. The observed hypermethylation is likely to be established by in trans interactions between homologous DNA sequences on different chromosomes in a dominant paternal manner. These findings confirm previous results that endosperm formation is, beside other factors, triggered by the alteration of methylation levels prior to fertilization. Furthermore, DNA methylation patterns can probably be transferred via the paternal gametes, influencing not only the epigenetic status of the sporophyte, but also of the following gametophyte and affecting seed development after subsequent fertilization.